The present invention relates to a silicon nitride filter suitable for removing dust, etc. contained in a high temperature exhaust gas, and a method for its production.
Heretofore, a cordierite type ceramic filter or silicon carbide type ceramic filter has been proposed as a filter to remove dust, etc. contained in a high temperature exhaust gas. However, the cordierite type ceramic filter is not necessarily adequate from the viewpoint of heat resistance and corrosion resistance although it is excellent in thermal shock resistance, and the silicon carbide type ceramic filter is not necessarily adequate with respect to the thermal shock resistance, although it is excellent in heat resistance and corrosion resistance.
Particularly when the ceramic filter is one intended for arresting diesel particulates (hereinafter referred to simply as particulates) discharged from a diesel engine (hereinafter referred to simply as an engine), it has been likely with the above-mentioned cordierite type filter or the silicon carbide type filter that the particulates arrested by the filter will locally burn to cause a melting loss, thus presenting a fatal damage to the ceramic filter. Further, the particulates contain a sulfur content and a phosphor content, whereby acid resistance is required, but the cordierite type filter used to be not necessarily adequate with respect to the acid resistance.
On the other hand, silicon nitride has excellent characteristics with respect to heat resistance, thermal shock resistance, corrosion resistance, acid resistance, mechanical strength, etc., and is expected to be useful as a filter for dust arresting or dust removing in a high temperature or corrosive environment. Especially, silicon nitride is excellent in heat resistance, thermal shock resistance, acid resistance and mechanical strength, and is thus considered to be a material suitable for a filter for particulates.
As a method for producing such a silicon nitride filter, several have been proposed.
For example, JP-A-6-256069 proposes a method of firing a green body comprising silicon nitride particles, clay and an oxide. Further, JP-A-7-187845, JP-A-8-59364 and JP-A-6-24859 propose methods of using as starting materials a mixture comprising silicon nitride particles and an organic silicon compound, a mixture comprising silicon nitride particles and a polysilazane and a mixture comprising silicon nitride particles and a synthetic resin foam, respectively. However, such methods of using silicon nitride particles as starting materials have had a problem that as compared with a method of using metal silicon particles as the starting material and converting it to silicon nitride by direct nitriding, pores with pore diameters of at most 1 xcexcm are little, whereby the Young""s modulus is high, the thermal shock resistance tends to be poor, the production cost tends to be problematic since the silicon nitride particles are relatively expensive.
On the other hand, as a method of employing metal silicon particles, JP-A-1-188479 proposes a production method to obtain a porous product having a nitriding ratio of the metal silicon particles of at most 50%, by using as a starting material a mixture comprising metal silicon particles and silicon nitride particles. However, in this method, the nitriding ratio of the metal silicon particles is at most 50%, whereby there will be a substantial amount of silicon metal remaining in the silicon nitride sintered body in the form of metal silicon without being nitrided, whereby there will be a problem such that excellent heat resistance or corrosion resistance of silicon nitride will be impaired.
Further, by the method of using metal silicon particles, sintering of the formed silicon nitride particles is usually not sufficient, whereby the mechanical strength of the porous body thereby obtained, tends to be inadequate.
The present invention provides a method for producing a silicon nitride filter, which comprises heat-treating in nitrogen a green body comprising from 40 to 90 mass % (hereinafter referred to simply as %) of metal silicon particles having an average particle diameter of from 1 to 200 xcexcm and from 10 to 60% of a pore-forming agent, provided that the total amount of the metal silicon particles and the pore-forming agent is at least 90%, to form a porous product made substantially of silicon nitride.
Another invention of the present invention provides a silicon nitride filter characterized in that the porosity is from 40 to 70%, and the cumulative pore volume of pores with diameters of at most 1 xcexcm is from 1 to 15 vol % of the total pore volume.
In the method for producing a silicon nitride filter of the present invention, a green body is used which comprises from 10 to 60% of a pore-forming agent and from 40 to 90% of metal silicon particles having an average particle diameter of from 1 to 200 xcexcm, provided that the total amount of the metal silicon particles and the pore-forming agent is at least 90%.
If the pore-forming agent is less than 10%, the proportion of pores to perform a filter function tends to be inadequate, and if the pore-forming agent exceeds 60%, no adequate strength tends to be obtained, although the porosity of the filter becomes large. Further, if the average particle diameter of the metal silicon particles is less than 1 xcexcm, the amount of moisture or oxygen adsorbed from outside air during e.g. preparation of the green body tends to increase, and when heat treated, the metal silicon particles tend to be oxidized before being nitrided, whereby the amount of silicon dioxide formed, tends to be too large. Further, if the average particle diameter of the metal silicon particles exceeds 200 xcexcm, metal silicon particles not nitrided tend to remain in the interior of the sintered body even after the heat treatment, whereby the properties as the silicon nitride filter tend to deteriorate. If the metal silicon particles are less than 40%, the merits of using metal silicon particles, i.e. the merit of using the direct nitriding reaction of metal silicon, will not be utilized. On the other hand, if the content of the metal silicon particles exceeds 90%, the content of the pore-forming agent tends to be small, whereby the porosity cannot be made large. The purity of the metal silicon particles may suitably be selected depending upon the purpose and application.
In this specification, the pore-forming agent is not particularly limited so long as it forms pores. The pore-forming agent may, for example, be one which flies upon e.g. decomposition during heat treatment, to form pores (hereinafter referred to as a flying-type pore-forming agent) or oxide ceramic hollow particles.
The heat-treating conditions are preferably two-step heat treatment in a nitrogen atmosphere, which is divided into a first step suitable for nitriding metal silicon particles and a second step suitable for sintering silicon nitride particles as formed nitride.
As the heat-treating conditions of the first step, it is preferred to maintain in a nitrogen atmosphere from 1000 to 1400xc2x0 C. for from 4 to 24 hours. If the temperature is lower than 1000xc2x0 C., nitriding of metal silicon particles tends to hardly take place. On the other hand, if the temperature exceeds 1400xc2x0 C., the metal silicon particles will melt in the vicinity of the melting point (1410xc2x0 C.) of metal silicon, whereby the shape cannot be maintained, such being undesirable. If the temperature maintaining time is less than 4 hours, nitriding of metal silicon particles tends to be inadequate, such being undesirable. Further, if the temperature maintaining time exceeds 24 hours, the nitriding reaction will no more substantially proceed, and the operation cost increases, such being undesirable.
As heat-treating conditions of the second step, it is preferred to maintain in a nitrogen atmosphere at from 1450 to 1800xc2x0 C. for from 1 to 12 hours. If the temperature is lower than 1450xc2x0 C., sintering of the silicon nitride particles tends to hardly proceed, such being undesirable, and if it exceeds 1800xc2x0 C., the silicon nitride particles tend to be decomposed, such being undesirable. If the temperature maintaining time is less than 1 hour, bonding of the particles to one another will not adequately proceed, such being undesirable. On the other hand, if it exceeds 12 hours, silicon nitride tends to be decomposed, such being undesirable. The heat treatment of the first step and the heat treatment of the second step may be carried out continuously without lowering the temperature or the temperature may be once lowered at an intermediate point.
The temperature raising rate at the time of the heat treatment may suitably be selected depending upon the size, shape, etc. of the green body. However, it is preferably from 50 to 600xc2x0 C./hr from the viewpoint of the nitriding ratio and the pore diameter. Even in a process of temperature raising, if the temperature is within a temperature range prescribed for the first step or the second step, the time passed will be included in the maintaining time for the first step or the second step.
Here, the nitrogen atmosphere is an atmosphere containing substantially solely nitrogen without containing oxygen, but it may contain other inert gas. The nitrogen partial pressure is preferably at least 50 kPa.
In the method for producing a silicon nitride filter of the present invention, the pore-forming agent is preferably oxide ceramic hollow particles. The method for producing a silicon nitride filter of the present invention wherein the pore-forming agent is oxide ceramics, will hereinafter be referred to as method 1.
In method 1, it is preferred to use a green body which comprises from 15 to 50% of oxide ceramic hollow particles and from 40 to 85% of metal silicon particles having an average particle diameter of from 5 to 200 xcexcm, provided that the total amount of the oxide ceramic hollow particles and the metal silicon particles, is at least 90%.
Oxide ceramic hollow particles (hereinafter referred to as hollow particles) may be any particles so long as they are capable of forming pores during the heat treatment and they serve as a sintering aid to the silicon nitride particles formed in the heat-treating step.
It is preferred that the hollow particles contain as the main component an oxide of at least one metal selected from the group consisting of Al, Si, Ca, Sr, Ba and Mg, since the effect as a sintering aid is thereby high.
The hollow particles may have outer skin portion being dense or porous so long as they are hollow. Further, the hollow particles are preferably spherical particles, as they are readily available, but particles other than spherical particles may be employed so long as they are hollow.
The average particle diameter of the hollow particles is preferably from 30 to 200 xcexcm, whereby the porosity of the filter to be obtained, will be large, and the strength will be secured. If the average particle diameter of the hollow particles is less than 30 xcexcm, the contribution to formation of pores will decrease. On the other hand, if the average particle diameter exceeds 200 xcexcm, the strength of the filter to be obtained tends to be inadequate, such being undesirable. The content of the hollow particles is preferably from 15 to 50% of the green body.
With the metal silicon particles to be used in method 1, the average particle diameter is preferably from 5 to 200 xcexcm, more preferably from 30 to 150 xcexcm.
In method 1, the total amount of the hollow particles and the metal silicon particles will be at least 90% in the green body.
In method 1, a common mixing means such as a ball mill or a mixer may be employed for mixing the hollow particles and the metal silicon particles, and a common ceramic-forming method such as press molding, extrusion molding or slip casting, may suitably be employed as a method for preparing a green body comprising hollow particles and metal silicon particles. Further, at the time of molding, an organic binder may be incorporated. As such an organic binder, an organic substance such as polyvinyl alcohol or its modified product, starch or its modified product, carboxymethylcellulose, hydroxylmethylcellulose, polyvinyl pyrrolidone, an acrylic resin or an acrylic copolymer, a vinyl acetate resin or a vinyl acetate copolymer, may, for example, be employed. The amount of such an organic binder is preferably from 1 to 10 parts by mass (hereinafter referred to simply as parts) per 100 parts of the green body.
As conditions for heat treating the above green body, it is preferred that the heat-treating conditions of the first step are to maintain the green body in a nitrogen atmosphere at from 1200 to 1400xc2x0 C. for from 4 to 12 hours, and the heat-treating conditions of the second step are to maintain it in a nitrogen atmosphere at from 1500 to 1800xc2x0 C. for 1 to 12 hours.
The porosity of the silicon nitride filter obtained by method 1, is preferably from 30 to 80%. The porosity is measured by an Archimedean method. If the porosity is less than 30%, the pressure loss tends to be large, such being undesirable as a filter. On the other hand, if the porosity exceeds 80%, the strength tends to be low, such being undesirable as a filter.
The average pore diameter as measured by a mercury immersion method of the silicon nitride filter obtained by method 1, is preferably from 5 to 40 xcexcm. If the average pore diameter is less than 5 xcexcm, the pressure loss during the use of the filter tends to be large, such being undesirable. If the average pore diameter exceeds 40 xcexcm. arresting and removing fine exhaust particles such as diesel particulates tend to be difficult, such being undesirable.
In the method for producing a silicon nitride filter of the present invention, the pore-forming agent is preferably a flying-type pore-forming agent. The method for producing a silicon nitride filter of the present invention wherein the pore-forming agent is a flying-type pore-forming agent, will be hereinafter referred to as method 2.
In method 2, it is preferred to use a green body which comprises from 10 to 50% of the flying-type pore-forming agent and from 40 to 90% of metal silicon particles having an average particle diameter of from 1 to 30 xcexcm, provided that the total amount of the flying-type pore-forming agent and the metal silicon particles is at least 90%.
As the flying-type pore-forming agent, either an organic substance or an inorganic substance may be suitably employed so long as it is capable of flying upon e.g. decomposition during the heat treatment, to form pores. It is preferred that the flying-type pore-forming agent is organic polymer particles, particularly heat decomposable polymer particles, since they will decompose and fly in the heat treatment process without leaving any residue in the sintered body, whereby properties of the obtainable silicon nitride porous product will not be impaired.
As such an organic polymer, polyvinyl alcohol, an acrylic resin, a vinyl acetate resin or cellulose, may, for example, be mentioned. If organic polymer particles added as a flying-type pore-forming agent during the temperature raising will not sufficiently be heat-decomposed in the temperature raising step in the heat treatment and will remain in a substantial amount as carbon, silicon carbide will be formed in the subsequent heat treating process, whereby pores are likely to be clogged, such being undesirable. From this viewpoint, it is preferred to use acrylic resin particles as a flying-type pore-forming agent, whereby it is readily heat decomposable, and the amount remaining as carbon will be little.
The content of the flying-type pore-forming agent is preferably from 10 to 50% in the green body, more preferably from 15 to 40%, whereby both the strength and porosity of the filter can be made high.
Further, it is particularly preferred that the flying-type pore-forming agent is spherical, whereby pores to be formed will also be spherical, and deterioration of the strength can be suppressed even when the porosity is made high. Further, when the flying-type pore-forming agent is spherical, the average particle diameter is preferably from 20 to 100 xcexcm. If the average particle diameter of the flying-type pore-forming agent is less than 20 xcexcm, the average pore diameter of the silicon nitride filter obtained after the heat treatment will be not higher than 5 xcexcm, such being undesirable. On the other hand, if it exceeds 100 xcexcm, the average pore diameter of the silicon nitride filter obtained after the heat treatment will be at least 20 xcexcm, such being undesirable as a filter for e.g. dusts.
The metal silicon particles to be used in method 2 preferably have an average particle diameter of from 1 to 30 xcexcm. The content of the metal silicon particles is preferably from 40 to 90%, more preferably from 50 to 80%, in the green body. In method 2, the total amount of the flying-type pore-forming agent and the metal silicon particles is at least 90% in the green body. If the total amount of the flying-type pore-forming agent and the metal silicon particles, is less than 90% in the green body, it is impossible to obtain a filter having the desired properties.
In method 2, as a method for forming a green body comprising the flying-type pore-forming agent and the metal silicon particles, a usual ceramic molding method as mentioned above, may suitably be employed. Further, at the time of the molding, an organic binder may be added separately from the flying-type pore-forming agent. As such an organic binder, the above-mentioned binder may preferably be employed. The amount of such an organic binder is preferably from 1 to 10 parts per 100 parts of the green body. Further, the flying-type pore-forming agent may serve as a binder for the green body.
In method 2, the heat-treating conditions of the first step are preferably such that the green body is maintained in a nitrogen atmosphere at from 1100 to 1400xc2x0 C. for from 5 to 24 hours. Further, the heat-treating conditions of the second step are preferably such that the green body is maintained in a nitrogen atmosphere at from 1450 to 1800xc2x0 C. for from 2 to 5 hours.
The porosity of the silicon nitride filter obtained by method 2, is preferably from 30 to 80%. The porosity is measured by an Archimedean method. If the porosity is less than 30%, the pressure loss becomes large, such being undesirable as a filter. Further, if the porosity exceeds 80%, the strength tends to be low, such being undesirable as a filter.
The average pore diameter as measured by a mercury immersion method of the silicon nitride filter obtained by method 2, is preferably from 5 to 20 xcexcm. If the average pore diameter is less than 5 xcexcm, the pressure loss during use of the filter tends to be large, such being undesirable. If the average pore diameter exceeds 20 xcexcm, it tends to be difficult to arrest and remove fine exhaust particles such as particulates, such being undesirable.
The ratio (hereinafter referred to as the nitriding ratio) of silicon contained as silicon nitride to total silicon of metal silicon of the silicon nitride filter obtained by method 2, is preferably at least 90%. If the nitriding ratio is less than 90%, the properties such as heat resistance and corrosion resistance of the silicon nitride filter will be low due to the remaining metal silicon particles, such being undesirable.
In this specification, the nitriding ratio of silicon nitride is calculated from the change in mass. Namely, the reaction for formation of silicon nitride is such that as shown by the formula 1, 3 mols of metal silicon will react with 2 mols of nitrogen to form 1 mol of silicon nitride.
3Si+2N2xe2x86x92Si3N4xe2x80x83xe2x80x83Formula 1
From the formula 1, if metal silicon is all converted to silicon nitride, the mass will be 1.67 times ((3xc3x97Si+4xc3x97N)/(3xc3x97Si)=(3xc3x9728+4xc3x9714)/(3xc3x9728)=1.67). If the change in mass is xcex1-times, the nitriding ratio is calculated from (xcex1xe2x88x921)/(1.67xe2x88x921)=(xcex1xe2x88x921)/0.67. For example, if it is 1.37 times, the nitriding ratio will be 55% (0.37/0.67xc3x97100=55%).
The silicon nitride filter of the present invention (hereinafter referred to as the present filter) is characterized in that the porosity is from 40 to 70%, and the cumulative pore volume of pores with diameters of at most 1 xcexcm is from 1 to 15 vol % of the total pore volume. The present filter preferably has a Young""s modulus of from 20 to 100 GPa and a thermal expansion coefficient of at most 4xc3x9710xe2x88x926/xc2x0 C. The thermal expansion coefficient is a value within a temperature range of from room temperature to 1000xc2x0 C.
The porosity of the present filter is from 40 to 70%. If the porosity is less than 40%, the pore volume will be too small, and the pressure loss will increase. On the other hand, if it exceeds 70%, the mechanical strength as a filter tends to be inadequate.
The ratio of the cumulative pore volume of pores with diameters of at most 1 xcexcm in the total pore volume (hereinafter referred to simply as a 1 xcexcm pore volume ratio) is from 1 to 15 vol %. If the 1 xcexcm pore volume ratio is less than 1 vol %, the Young""s modulus will be high, and the thermal shock resistance will deteriorate. Further, if the 1 xcexcm pore volume ratio exceeds 15 vol %, the pressure loss of the filter tends to increase, or the mechanical strength tends to be low. Preferably, the 1 xcexcm pore volume ratio is from 5 to 10 vol %.
The Young""s modulus of the present filter is preferably from 20 to 100 GPa. If the Young""s modulus is less than 20 GPa, the mechanical strength of the filter material tends to be too low. On the other hand, if it exceeds 100 GPa, the thermal stress generated by thermal shock tends to be large, whereby thermal shock resistance tends to deteriorate, such being undesirable.
In this specification, the pore volume is measured by a mercury immersion method, and the Young""s modulus is calculated from Young""s modulus E (Pa)="sgr"/xcex5, by measuring the stress "sgr" (Pa) and the strain xcex5, by the tensile strength measurements. The method for measuring the strain may, for example, be a method of using a strain gage.
For the measurement of the Young""s modulus, the sample size is 1xc3x971xc3x976 cm, and the longitudinal direction is the tensile direction. The tensile load is applied at 0.5 mm/min. In a case where the sample is a honeycomb, it is cut out so that the above-mentioned longitudinal direction will be the extrusion direction during the molding i.e. will be in parallel with the through holes, and at both ends, holes were firmly sealed with e.g. an acrylic resin adhesive or an epoxy type resin adhesive for from 5 to 10 mm from the end surfaces. The strain is measured by attaching a strain gage to the sample.
The above-mentioned method 1 or method 2 is preferably employed as a method for producing the present filter.